Request For Comments - RFC7626

Internet Engineering Task Force (IETF) S. Bortzmeyer
Request for Comments: 7626 AFNIC
Category: Informational August 2015
ISSN: 2070-1721
DNS Privacy Considerations
Abstract
This document describes the privacy issues associated with the use of
the DNS by Internet users. It is intended to be an analysis of the
present situation and does not prescribe solutions.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7626.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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turn, will refer to the example.com name servers. The example.com
name server will then return the answer. The root name servers, the
name servers of .com, and the name servers of example.com are called
authoritative name servers. It is important, when analyzing the
privacy issues, to remember that the question asked to all these name
servers is always the original question, not a derived question. The
question sent to the root name servers is "What are the AAAA records
for www.example.com?", not "What are the name servers of .com?". By
repeating the full question, instead of just the relevant part of the
question to the next in line, the DNS provides more information than
necessary to the name server.
Because DNS relies on caching heavily, the algorithm described just
above is actually a bit more complicated, and not all questions are
sent to the authoritative name servers. If a few seconds later the
stub resolver asks the recursive resolver, "What are the SRV records
of _xmpp-server._tcp.example.com?", the recursive resolver will
remember that it knows the name servers of example.com and will just
query them, bypassing the root and .com. Because there is typically
no caching in the stub resolver, the recursive resolver, unlike the
authoritative servers, sees all the DNS traffic. (Applications, like
web browsers, may have some form of caching that does not follow DNS
rules, for instance, because it may ignore the TTL. So, the
recursive resolver does not see all the name resolution activity.)
It should be noted that DNS recursive resolvers sometimes forward
requests to other recursive resolvers, typically bigger machines,
with a larger and more shared cache (and the query hierarchy can be
even deeper, with more than two levels of recursive resolvers). From
the point of view of privacy, these forwarders are like resolvers,
except that they do not see all of the requests being made (due to
caching in the first resolver).
Almost all this DNS traffic is currently sent in clear (unencrypted).
There are a few cases where there is some channel encryption, for
instance, in an IPsec VPN, at least between the stub resolver and the
resolver.
Today, almost all DNS queries are sent over UDP [thomas-ditl-tcp].
This has practical consequences when considering encryption of the
traffic as a possible privacy technique. Some encryption solutions
are only designed for TCP, not UDP.
Another important point to keep in mind when analyzing the privacy
issues of DNS is the fact that DNS requests received by a server are
triggered by different reasons. Let's assume an eavesdropper wants
to know which web page is viewed by a user. For a typical web page,
there are three sorts of DNS requests being issued:
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Primary request: this is the domain name in the URL that the user
typed, selected from a bookmark, or chose by clicking on an
hyperlink. Presumably, this is what is of interest for the
eavesdropper.
Secondary requests: these are the additional requests performed by
the user agent (here, the web browser) without any direct
involvement or knowledge of the user. For the Web, they are
triggered by embedded content, Cascading Style Sheets (CSS),
JavaScript code, embedded images, etc. In some cases, there can
be dozens of domain names in different contexts on a single web
page.
Tertiary requests: these are the additional requests performed by
the DNS system itself. For instance, if the answer to a query is
a referral to a set of name servers, and the glue records are not
returned, the resolver will have to do additional requests to turn
the name servers' names into IP addresses. Similarly, even if
glue records are returned, a careful recursive server will do
tertiary requests to verify the IP addresses of those records.
It can be noted also that, in the case of a typical web browser, more
DNS requests than strictly necessary are sent, for instance, to
prefetch resources that the user may query later or when
autocompleting the URL in the address bar. Both are a big privacy
concern since they may leak information even about non-explicit
actions. For instance, just reading a local HTML page, even without
selecting the hyperlinks, may trigger DNS requests.
For privacy-related terms, we will use the terminology from
[RFC6973].
2. Risks
This document focuses mostly on the study of privacy risks for the
end user (the one performing DNS requests). We consider the risks of
pervasive surveillance [RFC7258] as well as risks coming from a more
focused surveillance. Privacy risks for the holder of a zone (the
risk that someone gets the data) are discussed in [RFC5936] and
[RFC5155]. Non-privacy risks (such as cache poisoning) are out of
scope.
2.1. The Alleged Public Nature of DNS Data
It has long been claimed that "the data in the DNS is public". While
this sentence makes sense for an Internet-wide lookup system, there
are multiple facets to the data and metadata involved that deserve a
more detailed look. First, access control lists and private
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namespaces notwithstanding, the DNS operates under the assumption
that public-facing authoritative name servers will respond to "usual"
DNS queries for any zone they are authoritative for without further
authentication or authorization of the client (resolver). Due to the
lack of search capabilities, only a given QNAME will reveal the
resource records associated with that name (or that name's non-
existence). In other words: one needs to know what to ask for, in
order to receive a response. The zone transfer QTYPE [RFC5936] is
often blocked or restricted to authenticated/authorized access to
enforce this difference (and maybe for other reasons).
Another differentiation to be considered is between the DNS data
itself and a particular transaction (i.e., a DNS name lookup). DNS
data and the results of a DNS query are public, within the boundaries
described above, and may not have any confidentiality requirements.
However, the same is not true of a single transaction or a sequence
of transactions; that transaction is not / should not be public. A
typical example from outside the DNS world is: the web site of
Alcoholics Anonymous is public; the fact that you visit it should not
be.
2.2. Data in the DNS Request
The DNS request includes many fields, but two of them seem
particularly relevant for the privacy issues: the QNAME and the
source IP address. "source IP address" is used in a loose sense of
"source IP address + maybe source port", because the port is also in
the request and can be used to differentiate between several users
sharing an IP address (behind a Carrier-Grade NAT (CGN), for instance
[RFC6269]).
The QNAME is the full name sent by the user. It gives information
about what the user does ("What are the MX records of example.net?"
means he probably wants to send email to someone at example.net,
which may be a domain used by only a few persons and is therefore
very revealing about communication relationships). Some QNAMEs are
more sensitive than others. For instance, querying the A record of a
well-known web statistics domain reveals very little (everybody
visits web sites that use this analytics service), but querying the A
record of www.verybad.example where verybad.example is the domain of
an organization that some people find offensive or objectionable may
create more problems for the user. Also, sometimes, the QNAME embeds
the software one uses, which could be a privacy issue. For instance,
_ldap._tcp.Default-First-Site-Name._sites.gc._msdcs.example.org.
There are also some BitTorrent clients that query an SRV record for
_bittorrent-tracker._tcp.domain.example.
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Another important thing about the privacy of the QNAME is the future
usages. Today, the lack of privacy is an obstacle to putting
potentially sensitive or personally identifiable data in the DNS. At
the moment, your DNS traffic might reveal that you are doing email
but not with whom. If your Mail User Agent (MUA) starts looking up
Pretty Good Privacy (PGP) keys in the DNS [DANE-OPENPGPKEY], then
privacy becomes a lot more important. And email is just an example;
there would be other really interesting uses for a more privacy-
friendly DNS.
For the communication between the stub resolver and the recursive
resolver, the source IP address is the address of the user's machine.
Therefore, all the issues and warnings about collection of IP
addresses apply here. For the communication between the recursive
resolver and the authoritative name servers, the source IP address
has a different meaning; it does not have the same status as the
source address in an HTTP connection. It is now the IP address of
the recursive resolver that, in a way, "hides" the real user.
However, hiding does not always work. Sometimes [CLIENT-SUBNET] is
used (see its privacy analysis in [denis-edns-client-subnet]).
Sometimes the end user has a personal recursive resolver on her
machine. In both cases, the IP address is as sensitive as it is for
HTTP [sidn-entrada].
A note about IP addresses: there is currently no IETF document that
describes in detail all the privacy issues around IP addressing. In
the meantime, the discussion here is intended to include both IPv4
and IPv6 source addresses. For a number of reasons, their assignment
and utilization characteristics are different, which may have
implications for details of information leakage associated with the
collection of source addresses. (For example, a specific IPv6 source
address seen on the public Internet is less likely than an IPv4
address to originate behind a CGN or other NAT.) However, for both
IPv4 and IPv6 addresses, it's important to note that source addresses
are propagated with queries and comprise metadata about the host,
user, or application that originated them.
2.3. Cache Snooping
The content of recursive resolvers' caches can reveal data about the
clients using it (the privacy risks depend on the number of clients).
This information can sometimes be examined by sending DNS queries
with RD=0 to inspect cache content, particularly looking at the DNS
TTLs [grangeia.snooping]. Since this also is a reconnaissance
technique for subsequent cache poisoning attacks, some counter
measures have already been developed and deployed.
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2.4. On the Wire
DNS traffic can be seen by an eavesdropper like any other traffic.
It is typically not encrypted. (DNSSEC, specified in [RFC4033],
explicitly excludes confidentiality from its goals.) So, if an
initiator starts an HTTPS communication with a recipient, while the
HTTP traffic will be encrypted, the DNS exchange prior to it will not
be. When other protocols will become more and more privacy-aware and
secured against surveillance, the DNS may become "the weakest link"
in privacy.
An important specificity of the DNS traffic is that it may take a
different path than the communication between the initiator and the
recipient. For instance, an eavesdropper may be unable to tap the
wire between the initiator and the recipient but may have access to
the wire going to the recursive resolver, or to the authoritative
name servers.
The best place to tap, from an eavesdropper's point of view, is
clearly between the stub resolvers and the recursive resolvers,
because traffic is not limited by DNS caching.
The attack surface between the stub resolver and the rest of the
world can vary widely depending upon how the end user's computer is
configured. By order of increasing attack surface:
The recursive resolver can be on the end user's computer. In
(currently) a small number of cases, individuals may choose to
operate their own DNS resolver on their local machine. In this
case, the attack surface for the connection between the stub
resolver and the caching resolver is limited to that single
machine.
The recursive resolver may be at the local network edge. For
many/most enterprise networks and for some residential users, the
caching resolver may exist on a server at the edge of the local
network. In this case, the attack surface is the local network.
Note that in large enterprise networks, the DNS resolver may not
be located at the edge of the local network but rather at the edge
of the overall enterprise network. In this case, the enterprise
network could be thought of as similar to the Internet Access
Provider (IAP) network referenced below.
The recursive resolver can be in the IAP premises. For most
residential users and potentially other networks, the typical case
is for the end user's computer to be configured (typically
automatically through DHCP) with the addresses of the DNS
recursive resolvers at the IAP. The attack surface for on-the-
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wire attacks is therefore from the end-user system across the
local network and across the IAP network to the IAP's recursive
resolvers.
The recursive resolver can be a public DNS service. Some machines
may be configured to use public DNS resolvers such as those
operated today by Google Public DNS or OpenDNS. The end user may
have configured their machine to use these DNS recursive resolvers
themselves -- or their IAP may have chosen to use the public DNS
resolvers rather than operating their own resolvers. In this
case, the attack surface is the entire public Internet between the
end user's connection and the public DNS service.
2.5. In the Servers
Using the terminology of [RFC6973], the DNS servers (recursive
resolvers and authoritative servers) are enablers: they facilitate
communication between an initiator and a recipient without being
directly in the communications path. As a result, they are often
forgotten in risk analysis. But, to quote again [RFC6973], "Although
[...] enablers may not generally be considered as attackers, they may
all pose privacy threats (depending on the context) because they are
able to observe, collect, process, and transfer privacy-relevant
data." In [RFC6973] parlance, enablers become observers when they
start collecting data.
Many programs exist to collect and analyze DNS data at the servers --
from the "query log" of some programs like BIND to tcpdump and more
sophisticated programs like PacketQ [packetq] [packetq-list] and
DNSmezzo [dnsmezzo]. The organization managing the DNS server can
use this data itself, or it can be part of a surveillance program
like PRISM [prism] and pass data to an outside observer.
Sometimes, this data is kept for a long time and/or distributed to
third parties for research purposes [ditl] [day-at-root], security
analysis, or surveillance tasks. These uses are sometimes under some
sort of contract, with various limitations, for instance, on
redistribution, given the sensitive nature of the data. Also, there
are observation points in the network that gather DNS data and then
make it accessible to third parties for research or security purposes
("passive DNS" [passive-dns]).
2.5.1. In the Recursive Resolvers
Recursive Resolvers see all the traffic since there is typically no
caching before them. To summarize: your recursive resolver knows a
lot about you. The resolver of a large IAP, or a large public
resolver, can collect data from many users. You may get an idea of
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the data collected by reading the privacy policy of a big public
resolver, e.g., <https://developers.google.com/speed/public-dns/
privacy>.
2.5.2. In the Authoritative Name Servers
Unlike what happens for recursive resolvers, observation capabilities
of authoritative name servers are limited by caching; they see only
the requests for which the answer was not in the cache. For
aggregated statistics ("What is the percentage of LOC queries?"),
this is sufficient, but it prevents an observer from seeing
everything. Still, the authoritative name servers see a part of the
traffic, and this subset may be sufficient to violate some privacy
expectations.
Also, the end user typically has some legal/contractual link with the
recursive resolver (he has chosen the IAP, or he has chosen to use a
given public resolver), while having no control and perhaps no
awareness of the role of the authoritative name servers and their
observation abilities.
As noted before, using a local resolver or a resolver close to the
machine decreases the attack surface for an on-the-wire eavesdropper.
But it may decrease privacy against an observer located on an
authoritative name server. This authoritative name server will see
the IP address of the end client instead of the address of a big
recursive resolver shared by many users.
This "protection", when using a large resolver with many clients, is
no longer present if [CLIENT-SUBNET] is used because, in this case,
the authoritative name server sees the original IP address (or
prefix, depending on the setup).
As of today, all the instances of one root name server, L-root,
receive together around 50,000 queries per second. While most of it
is "junk" (errors on the Top-Level Domain (TLD) name), it gives an
idea of the amount of big data that pours into name servers. (And
even "junk" can leak information; for instance, if there is a typing
error in the TLD, the user will send data to a TLD that is not the
usual one.)
Many domains, including TLDs, are partially hosted by third-party
servers, sometimes in a different country. The contracts between the
domain manager and these servers may or may not take privacy into
account. Whatever the contract, the third-party hoster may be honest
or not but, in any case, it will have to follow its local laws. So,
requests to a given ccTLD may go to servers managed by organizations
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outside of the ccTLD's country. End users may not anticipate that,
when doing a security analysis.
Also, it seems (see the survey described in [aeris-dns]) that there
is a strong concentration of authoritative name servers among
"popular" domains (such as the Alexa Top N list). For instance,
among the Alexa Top 100K, one DNS provider hosts today 10% of the
domains. The ten most important DNS providers host together one
third of the domains. With the control (or the ability to sniff the
traffic) of a few name servers, you can gather a lot of information.
2.5.3. Rogue Servers
The previous paragraphs discussed DNS privacy, assuming that all the
traffic was directed to the intended servers and that the potential
attacker was purely passive. But, in reality, we can have active
attackers redirecting the traffic, not to change it but just to
observe it.
For instance, a rogue DHCP server, or a trusted DHCP server that has
had its configuration altered by malicious parties, can direct you to
a rogue recursive resolver. Most of the time, it seems to be done to
divert traffic by providing lies for some domain names. But it could
be used just to capture the traffic and gather information about you.
Other attacks, besides using DHCP, are possible. The traffic from a
DNS client to a DNS server can be intercepted along its way from
originator to intended source, for instance, by transparent DNS
proxies in the network that will divert the traffic intended for a
legitimate DNS server. This rogue server can masquerade as the
intended server and respond with data to the client. (Rogue servers
that inject malicious data are possible, but it is a separate problem
not relevant to privacy.) A rogue server may respond correctly for a
long period of time, thereby foregoing detection. This may be done
for what could be claimed to be good reasons, such as optimization or
caching, but it leads to a reduction of privacy compared to if there
was no attacker present. Also, malware like DNSchanger [dnschanger]
can change the recursive resolver in the machine's configuration, or
the routing itself can be subverted (for instance,
[ripe-atlas-turkey]).
A practical consequence of this section is that solutions for DNS
privacy may have to address authentication of the server, not just
passive sniffing.
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2.6. Re-identification and Other Inferences
An observer has access not only to the data he/she directly collects
but also to the results of various inferences about this data.
For instance, a user can be re-identified via DNS queries. If the
adversary knows a user's identity and can watch their DNS queries for
a period, then that same adversary may be able to re-identify the
user solely based on their pattern of DNS queries later on regardless
of the location from which the user makes those queries. For
example, one study [herrmann-reidentification] found that such re-
identification is possible so that "73.1% of all day-to-day links
were correctly established, i.e. user u was either re-identified
unambiguously (1) or the classifier correctly reported that u was not
present on day t+1 any more (2)." While that study related to web
browsing behavior, equally characteristic patterns may be produced
even in machine-to-machine communications or without a user taking
specific actions, e.g., at reboot time if a characteristic set of
services are accessed by the device.
For instance, one could imagine that an intelligence agency
identifies people going to a site by putting in a very long DNS name
and looking for queries of a specific length. Such traffic analysis
could weaken some privacy solutions.
The IAB privacy and security program also have a work in progress
[RFC7624] that considers such inference-based attacks in a more
general framework.
2.7. More Information
Useful background information can also be found in [tor-leak] (about
the risk of privacy leak through DNS) and in a few academic papers:
[yanbin-tsudik], [castillo-garcia], [fangming-hori-sakurai], and
[federrath-fuchs-herrmann-piosecny].
3. Actual "Attacks"
A very quick examination of DNS traffic may lead to the false
conclusion that extracting the needle from the haystack is difficult.
"Interesting" primary DNS requests are mixed with useless (for the
eavesdropper) secondary and tertiary requests (see the terminology in
Section 1). But, in this time of "big data" processing, powerful
techniques now exist to get from the raw data to what the
eavesdropper is actually interested in.
Many research papers about malware detection use DNS traffic to
detect "abnormal" behavior that can be traced back to the activity of
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malware on infected machines. Yes, this research was done for the
good, but technically it is a privacy attack and it demonstrates the
power of the observation of DNS traffic. See [dns-footprint],
[dagon-malware], and [darkreading-dns].
Passive DNS systems [passive-dns] allow reconstruction of the data of
sometimes an entire zone. They are used for many reasons -- some
good, some bad. Well-known passive DNS systems keep only the DNS
responses, and not the source IP address of the client, precisely for
privacy reasons. Other passive DNS systems may not be so careful.
And there is still the potential problems with revealing QNAMEs.
The revelations (from the Edward Snowden documents, which were leaked
from the National Security Agency (NSA)) of the MORECOWBELL
surveillance program [morecowbell], which uses the DNS, both
passively and actively, to surreptitiously gather information about
the users, is another good example showing that the lack of privacy
protections in the DNS is actively exploited.
4. Legalities
To our knowledge, there are no specific privacy laws for DNS data, in
any country. Interpreting general privacy laws like
[data-protection-directive] (European Union) in the context of DNS
traffic data is not an easy task, and we do not know a court
precedent here. See an interesting analysis in [sidn-entrada].
5. Security Considerations
This document is entirely about security, more precisely privacy. It
just lays out the problem; it does not try to set requirements (with
the choices and compromises they imply), much less define solutions.
Possible solutions to the issues described here are discussed in
other documents (currently too many to all be mentioned); see, for
instance, [QNAME-MINIMIZATION] for the minimization of data or
[TLS-FOR-DNS] about encryption.
6. References
6.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
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